Option flags for custom properties (bpy.props)
The PROPORTIONAL flag is not related to Blender's Proportional Editing feature, but Multi-Number Button Editing. It's broken in 2.72 for custom properties however. 

There doesn't seem to be any documentation in the API docs about this particular option beside what you already found. The corresponding is_... properties seem to be the only documentation.


HIDDEN: True when the property is hidden.See also Hide custom properties in the UI panel with python
SKIP_SAVE: True when the property is not saved in presets. See also this comment
ANIMATABLE: Property is animatable through RNA (should actually be true even if not specified)
LIBRARY_EDITABLE: Property is editable from linked instances (changes not saved)
ENUM_FLAG: Property is an EnumProperty and multiple entries can be selected (ShiftLMB)